Method of fabricating a cathode ray tube



June 29, I965 D. M. LINN 3,192,005

METHOD OF FABRIGATING A CATHODE RAY TUBE Filed Oct. 1, 1962 INVENTOR.

United States Patent "cc i 3,192,005 METHOD OF FABRICATING A CATHODE RAY TUBE Donald Max Linn, Marion, Ind., assignor to Radio Corporation of America, a corporation of Delaware Filed Oct. 1, 1962, Ser; No. 227,272 5 Claims. (Cl. 3161) This invention relates to cathode ray tubes and particularly to a method of fabricating such tubes.

. Electron guns of cathode ray tubes used, for example, in home television receivers usually comprise a plurality of axially aligned electrodes. These electrodes include a centrally apertured control grid cup and a tubular cathode having an end wall coated with electron emissive material and closely spaced opposite the aperture of the grid cup. Only a small central portion of the cathode emissive wall overlies the grid aperture and emits electrons which contribtue to the formation of the electron beam. In some tubes, such as commercial types 19AIP4, 19CHP4, '19CKP4, and 23DAP4, the cathode-to-control grid spacing may be only a few mils. In other types of tubes of the so-called low drive variety, the cathode-to-control grid spacing may be even less than a mil. Often times the cathode coating, which comprises a mixture of powders, is of irregular thickness and includes peaks.

In such cases the peaks might be spaced very close to the control grid and cause high resistance leakage between the cathode and grid. In severe cases of such peaks, complete cathode-to-grid short cicruits may result. Even when the electron guns have been fabricated by trained workmen using the best known .prior art techniques and equipment, as high as 15 to 40 percent scrap has resulted because of cathode-to-grid leakage or shorts in the abovenoted tube types.

It is therefore an object of this invention to provide a cathode ray tube having an electron gun which includes a new and improved cathode-grid structure and a method of fabricating such structure which reduces the occurrence of cathode-grid shorts and objectionable leakage,

without, at the same time, introducing other undesirable conditions.

In fabricating a cathode ray tube in accordance with this invention, the end wall of a tubular cathode is first bulged or domed outwardly and is'then coated with a suitable electron emissive material. The cathode is then mounted relative to the control grid and is fixed in selected spaced relationship therewith. By virtue of the domed characteristic. of the emissively coated cathode .wall, the peripheral areas of the emissive coating are spaced farther from the control grid than is the central portion of the coating which overlies the control grid aperture. Thus, peaks which occur in the peripheral areas of the coating do not cause shorts or high resistance leakage.

' In the drawings:

FIG. 1 is a side elevation view partly in section and with parts broken away' of a cathode ray'tube embodying the invention; and a FIG. 2 is an enlarged detail in section of the cathode ray tube of FIG. '1. 4 i Y The invention may be embodied in fabricating a cath- 19 may be of any suitable type such as one comprising a layer 20 of a phosphor material on the faceplate with a. superimposed film 21 of evaporated aluminum thereupon.

3,192.,M5 Patented June 29, 1965 The electron gun 18 comprises a plurality of coaxial, tubular, centrally-apertured electrodes including a control electrode cup 22, a screen electrode cup 24, and a focussing system including a first anode 26, a focussing ring 28, and a second anode 30. These electrodes are mounted in coaxial spaced relationship along a pair of insulator rods 32 by U-shaped mounting studs 34 which are fixed to the electrodes and embedded in the insulator rods. A cathode assembly 35 which includes a tubular cathode 36 mounted in a centrally apertured insulator disk 33, e.g., a ceramic washer, is coaxially mounted within the control electrode cup 22. The cathode 36 is closed at one end with a dome-shaped end wall 40 which is coated with suitable materials to provide an electron emissive surface. A heater filament 41 is provided within the tubular cathode 36.

A conductive coating 42 on the internal surface of the funnel 16 is connected to the luminescent screen 19 and to the second anode 30 through a plurality of spring snubbers 44 attached'to the second anode. A high voltage contact terminal indicated schematically by the arrow 46 is provided for applying a suitable voltageto the coating 42, the anode 30, the phosphor screen 19, and the anode 26 which may be connected (not shown) to the anode 30 internally of the tube. a 1

The neck 12 is closed at its distal end with a stern structure (not shown) over which a base 48 is fixed. A plurality of lead-in conductors 50 for applying suitable voltages to electrodes of the electron gun 18 are scaled through the stern and extend through the base 48.

Referring to FIG. 2, the control grid cup'22 comprises acylindrical wall 51 which is closed at one end with a planar wall 52 The end wall 52 has an electron beam aperture 54 centrally thereof. The cathode assembly 35 comprises the cathode36, the ceramic washer 38, and a pair of flanged retaining rims 56 and 58. The tubular cathode 36 is secured within the central aperture of the ceramic washer 38by a pair of circumferentially crimped beads 60 which are flattened against the opposite faces of :the Washer. The ceramic washer 38 is .held captive between the flanges of the flanged outer rim 56 and the flanged inner rim'58 which fits snuggly radially within the outer rim 56 and which is fixed thereto such as by welding. The end wall 40 of the cathode is provided with a coating 61 of suitable electron emissive material. The cathode assembly 35 is positioned as a unit within the grid cup 22 to obtain a desired spacing of the domeshaped end wall 49 of the cathode from the apertured end wall 52 of the grid cup. Electrical welds are then made to fix the cylindrical walls 62 and 64 of the rims 56 and 58 to the inside surface of the cylindrical wall 51 of the cathode cup 22.

As shown in FIG. 2, the emissive coating 61, by virtue of its being made of powdered materials, may include undesirable non-uniformities of thickness, such as peaks .70. By virtue of the domed shape of the end wall 46, the peak 70 is spaced from the grid wall 52. Yet, because'only a small central portion ofthe end wall 40 overlies the grid aperture 54, the fact that peripheral portions of the emissive end wall 40 are recessed from the grid wall 52 does not adversely affect the emission characteristics of the cathode or the electron beam formation by the grid. 7

In fabricating the cathode-grid structure of FIG. 2, a cathode sleeve with the emissive coating 61 not yet having been applied thereto, is first formed to provide the domed end wall 40. The emissive coating 61 is then applied to the convex surface of the domed Wall 46, such as by spraying, according to known practices. The coated cathode sleeve36 is secured within the aperture of the ceramic disk 38 by forming the crimped beads 60 therein. The

pair of retaining rims 56 and 58 are fitted to the ceramic disk 38 and the rims welded together. The securing of the cathode and of the retaining rings to the ceramic washer may either precede or follow the cathode forming and coating steps.

The cathode assembly 35 may be mounted within the grid cup 22 either before or after the grid cup 22 is mounted on the insulator support rods 32 together with the other electrodes of the electron gun 18. According to a preferred practice, the cathode assembly 35 is mounted within the grid cup 22 after the grid cup has been mounted on the insulator support rods. In following such preferred practice, spacing of the emissive coating 61 from the grid aperture 54 is accomplished by the use of a capacitance measuring technique. In accordance with this technique, the cathode assembly 35 is telescopically slid into the grid cup 22 while a capacitance measurement is taken between the cathode and another of the electrodes, such as the control grid 22 or, preferably, the screen grid cup 24. When a predetermined capacitance corresponding to a desired cathode-grid spacing is obtained, the cathode assembly 35 is fixed relative to the grid cup 22 by welding the retaining rims 56 and 58 to the grid wall 51.

The prior art teaches pushing or bending, as by dimpling an emissive coated cathode end wall toward the apertured wall of a control grid cup. However, such dimpling is done as a final cathode positioning step for the purpose of obtaining the desired cathode-grid spac ing and is done after the emissive coating has been applied to the cathode sleeve. This dimpling is so small as to provide an insignificant improvement in reduction of cathode-grid shorts or leakage as hereinbefore described. If such prior art dimpling were to be performed to a degree sufficient to obtain a substantial reduction in occurrence of leakage or shorts, the dimpling would cause cracking and consequent peeling of the previously applied emissive coating, which at the time of the dimpling, is dried and brittle.

What is claimed is:

1. The method of fabricating a cathode ray tube having an electron gun cathode-grid assembly of the closespaced variety in which an electron emissive coating is applied to the cathode by a process which often results in an irregular coating thickness that increases the likelihood of electrical leakage between the cathode and grid, said method comprising the steps of:

(a) outwardly doming a transverse end wall of a tubular cathode sleeve whereby to reduce in the peripheral areas of said end wall the likelihood of electrical cathode-grid leakage due to a subsequently applied electron emissive coating thereto which may possible have thickness irregularities, and then (b) coating the domed wall of said cathode sleeve on the outer convex side thereof with electron emissive material by said process,

(c) positioning said cathode sleeve very close to a centrally apertured grid cup, and

(d) fixing said cathode sleeve and said grid cup relative to each other.

2. The method of fabricating a cathode ray tube having an electron gun with a close-spaced cathode-grid assembly of the type comprising an apertured plate-like grid member and a tubular cathode sleeve having at one end thereof a transverse wall disposed opposite the aperture of said grid member and coated with electron emissive material by a process which often results in an irregular coating thickness that increases the likelihood of electrical leakage between the cathode and grid, said method comprising the steps of:

(a) outwardly doming said transverse wall of said cathode sleeve,

(b) then coating the resulting domed wall with electron emissive material by said process,

(c) selectively spacing said cathode relative to and within a few mils of said grid so that the peripheral areas of said domed wall are spaced farther from said grid than is the central area of said domed wall whereby the likelihood of electrical cathode-grid leakage due to possible coating irregularities in said peripheral areas is reduced, and

(d) fixing said cathode and said grid in a selected relative disposition.

3. The method of fabricating a cathode ray tube having an electron gun with a closed-spaced cathode-grid assembly of the type comprising a centrally apertured control grid cup and a tubular cathode sleeve having an end wall coated with electron emissive material by a process which often results in an irregular coating thickness that increases the likelihood of electrical leakage between the cathode and grid, said coated wall being spaced close to and coaxially with the aperture of said grid cup, said method comprising the steps of:

(a) outwardly doming said end wall of said cathode sleeve before it has been coated with said electron emissive material whereby to reduce in the peripheral areas of said end wall the likelihood of electrical cathode-grid leakage due to a subsequently applied electron emissive coating thereto which may possibly have thickness irregularities, and then (b) coating said domed wall on the outer convex side thereof with said electron emissive material by said process,

(c) positioning the emissive coated domed wall within a few mils of said aperture co-axial therewith, and

(d) fixing said cathode and said grid relative to each other in said coaxially close-spaced disposition.

4. The method of fabricating a cathode ray tube having an electron gun including a closed-spaced cathode-grid assembly in which an electron emissive coating is applied to the cathode by a process which often results in an irregular coating thickness that increases the likelihood of electrical leakage between the cathode and grid, said method including the steps of:

(a) outwardly doming a transverse end wall of a tubular cathode sleeve while it is yet without a coating of electron emissive material thereon,

(b) securing said cathode sleeve within a ceramic washer by a pair of circumferential crimped beads,

(c) attaching an annular retaining rim to said washer at its outer periphery, and then (d) coating the domed wall of said cathode sleeve on the outer convex surface thereof with a suitable electron emissive material by said process,

(e) coaxially sliding the assembly of said cathode, washer, and retaining rim into a centrally apertured grid cup to position the central portion of said emissive coating opposite the aperture of said grid cup at and within a few mils thereof so that the peripheral areas of said domed wall are spaced farther from said grid than is the central area of said domed wall whereby the likelihood of electrical cathodegrid leakage due to possible coating irregularities in said peripheral areas is reduced, and

(f) fixing said assembly relative to said grid cup by welding said retaining rim to the side wall of said grid cup.

5. The method of fabricating a cathode ray tube having an electron gun including a close-spaced cathode grid assembly of the type comprising a centrally apertured control grid cup, a ceramic washer, a tubular cathode sleeve which has a domed end wall coated with electron emissive material by a process which often results in an irregular coating thickness that increases the likelihood of electrical cathode-grid leakage, said cathode being coaxially mounted within the aperture of said ceramic washer by circumferential crimped beads, and tubular retaining rim means coaxially surrounding said washer (e) coaxially sliding the assembly of said cathode, Washer, and retaining rim means into said grid cup to position the central portion of said emissive coating opposite the central aperture of said grid cup and within a few mils thereof so that the peripheral areas of said domed Wall are spaced farther from and fixed to said washer and to the internal surface of the side wall of said grid cup so as to support the central portion of said electron emissive coating coaxially opposite the aperture of said grid cup, said method including the steps of: 5 (-a) outwardly doming said end wall of said cathode sleeve while it is yet without said coating of electron emissive material thereon whereby to reduce in the peripheral areas of said end wall the likelihood of said grid than is the central area of said domed Wall whereby the likelihood of electrical cathode-grid leakage due to possible coating irregularities in said electrical cathode-grid leakage due to a subsequently 10 peripheral areas is reduced, and applied electron emissive coating thereto which may fixing Said assembly relative to Said grid p y possibly h thi k i l i i welding said rim means to said side wall of said grid (b) securing said cathode sleeve within said ceramic washer by said circumferential crimped beads, References Cited by the Examiner (c) attaching said tubular retaining rim means to said 15 UNITED STATES PATENTS ceramic washer, and then (d) applying said coating of electron emissive material to the domed wall of said cathode sleeve on the outer convex surface thereof by said process,

10/37 Weinhart 313-821 X 20 FRANK E. BAILEY, Primary Examiner. 

1. THE METHOD OF FABRICATING A CATHODE RAY TUVE HAVING AN ELECTRON GUN CATHODE-GRID ASSEMBLY OF THE CLOSESPACED VARIETY IN WHICH AN ELECTRON EMISSIVE COATING IS APPLIED TO THE CATHODE BY A PROCESS WHICH OFTEN RESULTS IN AN IRREGULAR COATING THICKNESS THAT INCREASES THE LIKELIHOOD OF ELECTRICAL LEAKAGE BETWEEN THE CATHODE AND GRID, SAID METHOD COMPRISING THE STEPS OF: (A) OUTWARDLY DOMING A TRANSVERSE END WALL OF A TUBULAR CATHODE SLEEVE WHEREBY TO REDUCE IN THE PERIPHERAL AREAS OF SAID END WALL THE LIKELIHOOD OF ELECTRICAL CATHODE-GRID LEAKAGE DUE TO A SUBSEQUENTLY APPLIED ELECTRON EMISSIVE COATING THERETO WHICH MAY POSSIBLE HAVE THICKNESS IRREGULARITIES, AND THEN (B) COATING THE DOMED WALL OF SAID CATHODE SLEEVE ON THE OUTER CONVEX SIDE THEREOF WITH ELECTRON EMISSIVE MATERIAL BY SAID PROCESS, (C) POSITIONING SAID CATHODE SLEEVE VERY CLOSE TO A CENTRALLY APERTURED GRID CUP, AND (D) FIXING SAID CATHODE SLEEVE AND SAID GRID CUP RELATIVE TO EACH OTHER. 